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No top end under load 1990 20hp

Mercury 20hp 1990

No power @ WOT. Bogs right down. Motor starts everytime, idles great and trolls fine. But when opened up starts sputtering. Here's the thing...in neutral the motor will rev up no problem. Changed a lot parts. I've almost given up, almost. Any help would be greatly appriected.
 
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Re: No top end under load

Jeff the carb has been cleaned and new parts. fuel pump too. I'm new to this but if it can be rev'd high in neutral how can it be a fuel problem?
 
Re: No top end under load

Jonny,
it takes a lot less gas to rev an engine when it's not under load. A carb that has a partially obstructed main jet(s) will oftentimes be able to supply enough fuel to rev the engine in neutral but not enough to bring it up to speed when actually pushing the boat.
 
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Re: No top end under load

Agree - think of your car - don't you have to step on it a little harder (give it more gas) when you are towing the boat up a hill?. Your outboard is the same. In neutral you can wind her right up and she is only sucking "a little" gas - drop it in gear and ask it to push 1000 pounds and it wants a "way bigger straw" and if that fuel line, filter or carb is clogged - OR the gas is old, lost it's octane, is now 5-10% water etc, she will bogg on you...

If you are positive that the carb was cleaned, rebuilt and adjusted properly (and the timing linkage wasn't messed up) then I would be looking at bad/old gas as the problem and possibly some water in the tank.
 
Re: No top end under load

that makes sense about the straw analogy. carb was clean when i took it apart. however I still cleaned a blew out everything. new gaskets all around. I have a gut feeling it could be the timing. But how can it get messed up. I have gone through about 4 tanks since problem started. Could running on the richer side cause this problem?
 
Re: No top end under load

Running rich can lead to an excess of carbon build-up in the cylinders, which can detract from top end performance (initially) and ultimately lead to piston/cylinder failure (at the extreme).

A can of PowerTune (5 bucks at any Merc dealer) will take care of the carbon (if present).

Timing does not go "out" by itself. If you haven't fiddled with it, and it was set correctly when you put the carb back on (or are positive you didn't disturb the linkage in the process), then the only way it could be off is if the linkage got damaged or the trigger got stuck to the base.

With the hood off, if it looks like it's advancing ok (not binding, the linkage isn't bending etc) and it was "ok" before, then it's most likely not the problem...
 
Re: No top end under load

I'm going to strip the carb again...hopefully it helps. I've never messed with the timing and nothing is binding. Its weird b/c it stopped running at WOT while I was in WOT. I can see fuel going into the intake but still boggs.
 
Re: No top end under load

Your previous cleaning could have loosened up some varnish just enough that it shook loose when you were at WOT and partially clogged the main jet.
 
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